(1) Field of the Invention
The present invention is directed to the production of activated carbon. More particularly, it is directed to the production of highly microporous activated carbon from a lignocellulosic material. Specifically, the present invention is directed to the production of highly microporous activated carbon from a lignocellulosic precursor by a two stage chemical activation process employing, sequentially, phosphoric acid and potassium hydroxide. One use of such activated carbon is in the adsorption of gases, including natural gas.
(2) Description of the Prior Art
Practical storage of natural gas for such uses as a vehicle fuel and the like involves portable containerization of the natural gas. Natural gas is a leading contender for use as an alternative fuel for automobiles, particularly in areas designated as "zero emission" zones under the 1990 Clean Air Act. The majority of natural gas vehicle's operating in the United States use compressed natural gas at pressures of up to about 3500 psi. However, low pressure storage systems are being developed in which natural gas is contained in cylinders packed with adsorbent material to achieve near-liquid methane density. Economic evaluations by the natural gas industry indicate that adsorbed natural gas (ANG) would be comparable economically with compressed natural gas (CNG) at a deliverable gas capacity of 150 volumes of gas per cylinder volume (v/v) at a storage pressure of 500 psi.
Natural gas, which is primarily methane, is adsorbed in pores and on surfaces of the adsorbent medium. Under such conditions, the adsorbed gas assumes properties not unlike those of its liquid state. Typical adsorbents are solids with pores and fissures throughout their structure. Methane molecules preferentially adsorb in pores having a diameter of about 10-15 Angstroms (.ANG.).
Active carbon long has been used for removal of impurities and recovery of useful substances from liquids and gases because of its high adsorptive capacity. Generally, "activation" refers to any of the various processes by which the pore structure is enhanced. Typical activation processes involve treatment of carbon sources--such as resin wastes, coal, coal coke, petroleum coke, and lignocellulosic materials including residues from pulp production, wood (like wood chips, sawdust, and wood flour), nut shell (like almond shell and coconut shell), kernel, and fruit stone (like olive and cherry stone)--either thermally (with an oxidizing gas) or chemically (usually with phosphoric acid or metal salts). Such activated carbons maintain the original macrostructure of the starting material and, therefore, a similar pore distribution of micropores of less than 20 .ANG. in width, mesopores of 20 .ANG. to 500 .ANG. (divided between small mesopores of 20 .ANG. to less than 50 .ANG. in width and large mesopores of 50 .ANG. to 500 .ANG. in width), and macropores of greater than 500 .ANG. in width.
Commercial ANG carbons currently provide up to 100 v/v at a storage pressure of 500 to 900 psi (see U.S. Pat. No. 4,522,159). It is highly desirable to provide 100 v/v or greater storage capacity due to limitations of inexpensive compressor technology. Storage pressures of 900 psi represent an upper limit due to DOT regulations regarding transport and testing of gas cylinders. As the surface area of an active carbon is proportional to the carbon is microporosity and since the methane adsorption capacity of an active carbon is enhanced by increasing its microporosity, activation methods are sought which are pore size specific to improve over current commercial ANG active carbon.
Citing disclosures of potassium hydroxide (KOH) activation of coal in U.S. Pat. Nos. 3,764,561 and 4,082,694, the patentees of U.S. Pat. No. 4,769,359 teach the production of active carbon which enables high adsorption of gases per unit volume by treating coal with a liquid mixture comprising KOH and sodium hydroxide (NaOH) and subsequently carbonizing at 500.degree.-800.degree. C. A method of producing activated carbon with a large surface area and a low sulfur content also is taught in U.S. Pat. No. 5,064,805 by mixing coconut shell char with melted potassium hydroxide hydrate at a temperature sufficiently high to cause activation. Also, U.S. Pat. No. 4,082,694 teaches solid KOH activation of specific carbonaceous feeds including coal, coal coke, and petroleum coke to produce cage-like microporous structures particularly useful for water purification.
Chemical activation of wood-based carbon with phosphoric acid (H.sub.3 PO.sub.4) is disclosed in U.S. Pat. No. Re. 31,093 to improve the carbon's decolorizing and gas adsorbing abilities. Also, U.S. Pat. No. 5,162,286 teaches phosphoric acid activation of wood-based material which is particularly dense and which contains a relatively high (30%) lignin content, such as nut shell, fruit stone, and kernel. Zinc chloride (ZnCl.sub.2) also is a common chemical activation agent. Phosphoric acid activation of lignocellulose material also is taught in U.S. Pat. No. 5,204,310 as a step in preparing carbons of high activity and high density.
None of these activated carbons or other known carbons, however, achieve the desired objective of providing 150 v/v of deliverable gas capacity at 500 psi. At this storage pressure, known commercial carbons have achieved, at best, about 120 v/v of deliverable gas capacity. Therefore, the objective of this invention is to provide a highly microporous activated carbon of improved capacity (i.e., greater than 120 v/v at 500 psi) and preferably, a further objective is that the carbon is capable of meeting the industry target (i.e., 150 v/v) for a deliverable capacity of natural gas stored on activated carbon at 500 psi. Also, it is an objective of this invention to provide a process for producing the highly microporous activated carbon. It is an even further objective of this invention to provide a method for storing natural gas at low pressure using the highly microporous activated carbon.